Prior art radar systems tended to use narrower pulses to detect and resolve smaller targets. The spatial resolution of radar pulses when propagating through space is equal to (c.times.T)/2, where T is the pulse width time and c is the speed of light. However, the power requirements of such radars increases when using narrower pulses so as to project sufficient energy for detection where the energy transmitted equal to the power times the pulse width, T. In order to avoid the use of high power systems with exceedingly narrow radar pulses, linear frequency modulation (LFM) chirp pulses having longer pulse widths, but lower power requirements, have been used. The reflected target signal is processed using Matched Filter Processing, Cross Correlation Processing, Spectrum Analyzer Processing, Stretched Pulse Compression Processing, and like compression methods, to provide improved spatial resolution of the targets. These radar transmission and compression processing methods require the use of a local LFM chirp generator.
The LFM chirp pulse is a frequency modulated pulse where the modulated frequency typically linearly increases from an initial frequency -B/2 to +B/2 over a finite time equaling a pulse width T, for example, from -20 MHz to +20 MHz, providing a 40 MHz bandwidth B, over the pulse width T, for example, 12.8 micro-seconds, modulating an intermediate center frequency, for example, 160 MHz. This modulated signal is typically stepped up and mixed to a higher radio frequency (RF) carrier prior to transmission. Various means have been employed to generate such linear frequency modulation chirp pulses. The LFM chirp pulse, modulating the intermediate frequency has a time-bandwidth product equal the pulse width time T multiplied by the bandwidth B, for example 12.8 micro-seconds times 40 MHz, or 512. LFM chirp pulses have been generated by various hardware means. Chief among them are Dispersive Delay Lines, Surface Acoustic Wave devices and Direct Digital Synthesis systems.
A Dispersive Delay Line device is a group of lumped circuit elements e.g. a group of the RCL circuits which generate a respective group of staggered delay signals which are summed together and which provide the LFM chirp pulse when excited by an impulse stimulus. A Surface Acoustic Wave (SAW) device is a metalized crystalline device that is also subjected to a high impulse signal to produce the linear frequency modulated chirp pulse. Due to parasitic and device geometry manufacturing limits, present Dispersive Delay Line and SAW devices provide output signals which are exceedingly attenuated due to insertion losses with concomitant LFM chirp pulse width and bandwidth limitations. Severely attenuated signals are subject to poor signal-to-noise ratios limiting the usefulness of the Dispersive Delay Line and SAW devices. The Dispersive Delay Line and SAW devices generate the LFM chirp pulse typically with limited time-bandwidth products of up to about 500.
Direct Digital Synthesis (DDS) methods of generating the LFM chirp pulse typically employ digital programmed memories having stored digitized sinusoidal values that are typically fed into a digital to analog converter, such that as the digital values are cycled into the D/A converter at an increasing rate for a certain pulse width time T, the analog converter produces the LFM chirp pulse through that pulse width. However, more power is required the faster the digital and analog circuits are operated. Intrinsic device characteristics and modest power requirements limit the speed and usefulness of such DDS systems. Present technology limitations of digital systems and analog circuits using the DDS is approaching 500 MHz switching speeds using Gallium-Arsenide processing technology, while suffering from relatively high power consumption, for example, 40 watts. While the DDS method may provide chirp pulse time-bandwidth products in excess of 500, such methods have unacceptably high power requirements.
Improved resolution of the reflected radar target signals is desired for more accurate radar target detection and recognition. The received LFM chirp pulse signal is compressed using conventional Match Filter Processing or Cross Correlation Processing methods to provide a high resolution detection. The compression methods act upon reflected received target signals to compress the received LFM chirp pulse signal into a compressed narrow pulse spike signal. The pulse width of the compressed pulse spike signal is commonly defined by its 3 db points. The pulse width of the compressed pulse spike signal is equal to c/2B where B is the bandwidth of the LFM chirp pulse signal, that is, the difference between -B/2 and +B/2, or simply B. Decreased pulse width of the compressed pulse spike increases the range resolution of the target, especially applicable where individual targets within a group of moving targets, are to be accurately detected. Narrowing the pulse widths of the compressed pulse spike increases the range resolution and possible density of such compressed pulse spikes, thus allowing for a more accurate image resolution of the target or multiple targets. Thus, it is desired to increase the bandwidth of the LFM chirp signal for improved target resolution.
Equally important, it is further desired to have an improved signal-to-noise ratio of the compressed pulse spike signal. The signal-to-noise ratio depends upon the energy in the signal that is transmitted. Increasing the transmit energy increases the energy of the signal that is reflected off the target and subsequently received. The energy of the signal transmitted is equal to the power level multiplied by the pulse width of the LFM chirp signal. The longer the pulse width, the more energy transmitted at a given power transmission level. Thus, increasing the pulse width of the transmitted signal, at a given power level, increases the signal-to-noise ratio of the compressed pulse spike signal. It is therefore desired to transmit longer LFM chirp pulses, in lieu of increasing the power transmission level.
The transmitted LFM chirp pulse has a time-bandwidth product (T)(B), which is a measure of merit and potential performance of a given radar system. The detection of a target by a radar system is enhanced by increasing the LFM chirp pulse width thereby increasing the signal-to-noise ratio of the compressed pulse spike signal, at a given power transmission level, and is enhanced by increasing the bandwidth thereby decreasing the compressed pulse spike pulse width time for increased range resolution. It is desirable to improve the time-bandwidth products of LFM chirp pulses, with a longer pulse width time T over a wider bandwidth B. The generation of LFM chirp pulses with time-bandwidth products greater than 10,000 is difficult and beyond present day technologies. Thus, there exists a continuing need for improved LFM chirp pulse generators having extended pulse widths and extended bandwidths producing large time-bandwidths products greater than 10,000, with low power requirements and within present day technology limitations. The problems of the prior art low time-bandwidth product of LFM chirp pulse generators are solved or reduced using the present invention.